The invention pertains to polymerization reactions, particularly to monitor and control the rate and the amount of conversion in such reactions. More particularly, the invention pertains to accurately determining the rate and amount of conversion at a particular moment in a polymerization reaction so as to control the rate of conversion and to optimize cooling resources.
Most polymerization reactions today are run open loop with respect to the product quality (end-use) properties. Also operations involved in the manufacturing process are scheduled by a simple timer without attention to the actual progress of reaction.
In the last decade, the affordability of powerful computers finally made it possible to exploit the advanced control concepts control theorists have been developing since the 1960's. As a result, control of continuous processes like refinery distillation columns or power generation units has seen a rapid evolution from single loop proportional, integral and differential (PID) controllers to multivariable predictive controllers with built-in constraint optimization whose performance cannot be matched by the old PID solutions.
For a number of reasons, this progress so far has avoided batch processes. Control wise, most batches are still run the way they were thirty or more years ago. If there was a change, it affected control hardware, but not control algorithms. A batch recipe still prescribes time profiles of temperatures or pressures to be followed by a batch reactor in order to make the product. Feedback controllers, usually PID's, are routinely used to make the batch track the recipe in the presence of variations in feedstock concentration and purity, catalyst activity, reactor fouling and so on.
Maintaining batch recipe temperatures and pressures is important but it should not be the control objective. After all, the process owner does not sell batch temperatures or pressures. They are mere process parameters and, by themselves, are not even sufficient ones. It is well known and exemplified below for the case of polymerization processes, that two batches with perfectly identical temperature and pressure profiles can still have different rates at which monomer is converted into polymer, and thus yield products with inconsistent quality. When comes to the end-use parameters of the real product, which are determining its marketable quality, most batch processes are still run open loop, with all the negative consequences that an open loop recipe execution entails.
With the present invention, that approach is replaced with a feedback controller for polymerization processes that closes the loop using a measurement directly tied to the product's marketable quality, and thus employs feedback to eliminate quality variations and inconsistencies due to the fluctuations of process inputs and operating conditions.
The invention is a polymerization control that allows the user to specify independently the reaction mixture temperature and the degree of monomer conversion profiles as a function of time, and execute them under feedback control. This both improves the run-to-run consistency of the product and reduces the uncertainty of the reaction time and coolant consumption at any given instant. Because the coolant availability often is the limiting factor of production capacity, the improved predictability of individual batch runs offers an opportunity to improve batch planning and scheduling and thus increase the plant yield without expensive retrofits.